diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
new file mode 100644
index 0000000..2a8c44a
--- /dev/null
+++ b/evaluator/evaluator.go
@@ -0,0 +1,34 @@
+package evaluator
+
+import (
+	"monkey/ast"
+	"monkey/object"
+)
+
+func Eval(node ast.Node) object.Object {
+	switch node := node.(type) {
+
+	// Statements
+	case *ast.Program:
+		return evalStatements(node.Statements)
+
+	case *ast.ExpressionStatement:
+		return Eval(node.Expression)
+
+	// Expressions
+	case *ast.IntegerLiteral:
+		return &object.Integer{Value: node.Value}
+	}
+
+	return nil
+}
+
+func evalStatements(stmts []ast.Statement) object.Object {
+	var result object.Object
+
+	for _, statement := range stmts {
+		result = Eval(statement)
+	}
+
+	return result
+}
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
new file mode 100644
index 0000000..0c6779c
--- /dev/null
+++ b/evaluator/evaluator_test.go
@@ -0,0 +1,45 @@
+package evaluator
+
+import (
+	"monkey/lexer"
+	"monkey/object"
+	"monkey/parser"
+	"testing"
+)
+
+func TestEvalIntegerExpression(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected int64
+	}{
+		{"5", 5},
+		{"10", 10},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		testIntegerObject(t, evaluated, tt.expected)
+	}
+}
+
+func testEval(input string) object.Object {
+	l := lexer.New(input)
+	p := parser.New(l)
+	program := p.ParseProgram()
+
+	return Eval(program)
+}
+
+func testIntegerObject(t *testing.T, obj object.Object, expected int64) bool {
+	result, ok := obj.(*object.Integer)
+	if !ok {
+		t.Errorf("object is not Integer. got=%T (%+v)", obj, obj)
+		return false
+	}
+	if result.Value != expected {
+		t.Errorf("object has wrong value. got=%d, want=%d", result.Value, expected)
+		return false
+	}
+
+	return true
+}
diff --git a/object/object.go b/object/object.go
new file mode 100644
index 0000000..ea095bc
--- /dev/null
+++ b/object/object.go
@@ -0,0 +1,48 @@
+package object
+
+import "fmt"
+
+type ObjectType string
+
+const (
+	INTEGER_OBJ = "INTEGER"
+	BOOLEAN_OBJ = "BOOLEAN"
+	NULL_OBJ    = "NULL"
+)
+
+type Object interface {
+	Type() ObjectType
+	Inspect() string
+}
+
+type Integer struct {
+	Value int64
+}
+
+func (i *Integer) Type() ObjectType {
+	return INTEGER_OBJ
+}
+func (i *Integer) Inspect() string {
+	return fmt.Sprintf("%d", i.Value)
+}
+
+type Boolean struct {
+	Value bool
+}
+
+func (b *Boolean) Type() ObjectType {
+	return BOOLEAN_OBJ
+}
+
+func (b *Boolean) Inspect() string {
+	return fmt.Sprintf("%t", b.Value)
+}
+
+type Null struct{}
+
+func (n *Null) Type() ObjectType {
+	return NULL_OBJ
+}
+func (n *Null) Inspect() string {
+	return "null"
+}
diff --git a/repl/repl.go b/repl/repl.go
index c0dc6d4..94d10e7 100644
--- a/repl/repl.go
+++ b/repl/repl.go
@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"fmt"
 	"io"
+	"monkey/evaluator"
 	"monkey/lexer"
 	"monkey/parser"
 )
@@ -30,8 +31,11 @@ func Start(in io.Reader, out io.Writer) {
 			continue
 		}
 
-		io.WriteString(out, program.String())
-		io.WriteString(out, "\n")
+		evaluated := evaluator.Eval(program)
+		if evaluated != nil {
+			io.WriteString(out, evaluated.Inspect())
+			io.WriteString(out, "\n")
+		}
 	}
 }